A power-steering system in a motor vehicle is designed to provide appropriate hydraulic or electrical steering assist to allow a driver of the vehicle to complete a turn thereof. The driver applies a steering input through a manual steering wheel of the vehicle that is rotationally connected to a first shaft, which, in turn, is rotationally coupled to a second shaft that is, in turn, connected to a steering mechanism. The first and second shafts are coupled and transmit torque to each other by a compliant member, such as a torsion bar. The torsion bar allows the first shaft to rotate with respect to the second shaft by a predetermined number of degrees. Mechanical stops prevent further rotation. The amount of the steering assist applied to the steering mechanism is determined as a function of the degree of torsional strain or movement in the torsion bar.
In a hydraulic-power steering system, the steering assist is controlled by a power-steering valve. Systems have been developed to vary a level of assist as a function of speed of the vehicle.
One example is a “variable effort” system that uses a magnetic actuator to vary torsional stiffness of the power-steering valve to change actuation, effort, or torque (referred to generically as “magnetic steer”). A control module determines magnitude and direction of current applied to an excitation coil of the magnetic actuator to vary the torque as a function of the vehicle speed.
In all of these systems, however, only a level of torque is variable. Advanced steering functions—such as active return, pull-compensation, lane-keeping, and park-assist—are not achievable.